linux-kernel - Re: [PATCH] KVM: Fix error path in kvm_vm_ioctl_create_vcpu() on xa

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Message-ID: <ZrPuC59gatPJ4UzQ@google.com>
Date: Wed, 7 Aug 2024 14:58:35 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Michal Luczaj <mhal@...x.co>
Cc: Will Deacon <will@...nel.org>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Paolo Bonzini <pbonzini@...hat.com>, Alexander Potapenko <glider@...gle.com>, Marc Zyngier <maz@...nel.org>
Subject: Re: [PATCH] KVM: Fix error path in kvm_vm_ioctl_create_vcpu() on
 xa_store() failure

On Tue, Aug 06, 2024, Michal Luczaj wrote:
> On 8/6/24 00:56, Sean Christopherson wrote:
> > [...]
> > +	/*
> > +	 * xa_store() should never fail, see xa_reserve() above.  Leak the vCPU
> > +	 * if the impossible happens, as userspace already has access to the
> > +	 * vCPU, i.e. freeing the vCPU before userspace puts its file reference
> > +	 * would trigger a use-after-free.
> > +	 */
> >  	if (KVM_BUG_ON(xa_store(&kvm->vcpu_array, vcpu->vcpu_idx, vcpu, 0), kvm)) {
> > -		r = -EINVAL;
> > -		goto kvm_put_xa_release;
> > +		mutex_unlock(&vcpu->mutex);
> > +		return -EINVAL;
> >  	}
> >  
> >  	/*
> > @@ -4302,6 +4310,7 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id)
> >  	 */
> >  	smp_wmb();
> >  	atomic_inc(&kvm->online_vcpus);
> > +	mutex_unlock(&vcpu->mutex);
> >  
> >  	mutex_unlock(&kvm->lock);
> >  	kvm_arch_vcpu_postcreate(vcpu);
> > @@ -4309,6 +4318,7 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id)
> >  	return r;
> >  
> >  kvm_put_xa_release:
> > +	mutex_unlock(&vcpu->mutex);
> >  	kvm_put_kvm_no_destroy(kvm);
> >  	xa_release(&kvm->vcpu_array, vcpu->vcpu_idx);
> 
> Since we're handling the impossible, isn't the BUG_ON part missing
> mutex_unlock(&kvm->lock)?

Doh, yes.